Synthesis and antimicrobial activity of N-alkyl substituted p-methyl (E)-3- and 4-azachalconium bromides

Twenty new N-alkyl substituted p-methyl (E) -3- and 4-azachalcones (1a-j, 2a-j) {3-[(1E)-3-(4-methylphenyl)- 3-oxoprop-1-en-1-yl]-1-alkyl ($C _{5 - 12 , 14 - 15}$) pyridinium bromides (1a-j) and 4-[(1E)-3-(4-methylphenyl)- 3-oxoprop-1-en-1-yl]-1-alkyl ($C _{5 - 12 , 14 - 15}$) pyridinium bromides (2a-j)} were synthesized and tested for antimicrobial activities against Staphylococcus aureus, Staphylococcus epidermidis, Bacillus subtilis, Enterococcus faecalis, Proteus vulgaris, and Escherichia coli. They showed good antimicrobial activity especially against the gram-positive bacteria tested with minimal inhibitory concentration (MIC) values less than 4.7 μg/mL in most cases. The optimum length of the alkyl chain for better and broader activity is situated in the range of 8-12 carbon atoms in the series of compounds 1a-j, 2a-j. The non-alkylated compounds 1-2 were not effective as were the ones alkylated with 14 or 15 C alkyl groups (1i, 1j, 2i, 2j). N-Alkyl derivatives of p-methyl (E) -3-azachalcone (1a-h) showed better activity in comparison to those of p-methyl (E)-4- azachalcone (2a-h). The antimicrobial activity increased as the length of the alkyl substitution increased from 5 to 12 carbons.